1. Field of the Invention
This invention relates to a transparent, electrically conductive film, and, in particular, to a transparent, electrically conductive film which is suitable for use as a transparent electrode in various optical devices, such as solar cells, optical image sensors sand liquid crystal display panels.
2. Description of the Prior Art
In prior art optical devices, such as solar cells, optical image sensors and liquid crystal display panels, an oxide-family transparent, electrically conductive film, such as In.sub.2 O.sub.3 and SnO.sub.2, has been formed on a transparent substrate of glass or polymer as a transparent electrode. On the other hand, research and development in optical devices using amorphous is silicon (hereinafter, also referred to as a-silicon) has been carried out very actively in recent years, and a-silicon optical devices, such as a-silicon solar cells, a-silicon optical image sensors, and display units using a-silicon as a light-emitting material, have become commercially available at least partly.
However, when use was made of an a-silicon family material, prior art transparent electrodes typically had a problem of deterioration of interface condition. That is, when manufacturing a film of a-silicon, it is common practice to rely on the glow discharge decomposition method using mono-silane (SiH.sub.4) as a source material. In this case, however, oxygen atoms contained in the above-mentioned oxide-family transparent, electrically conductive film formed as a transparent electrode on the substrate react with hydrogen radicals generated in plasma, thereby forming H.sub.2 O. The production of H.sub.2 O provides effects on the adhesively and/or surface state of a-silicon material, and thus there is a possibility that the interface condition of the resultant transparent electrode is significantly deteriorated. Moreover, even if a transparent electrode is formed, at the interface between In.sub.2 O.sub.3 and a-Si:H, for example, In, O, and Si diffuse from one another to form new compounds, such as SiO.sub.2, thereby hindering obtainment of an excellent interface, which could provide significant adverse effects on device characteristics.
As described above, prior art transparent electrodes were typically comprised of an oxide of semiconductor material, such as ITO (Indium Tin Oxide), SnO.sub.2, SnO.sub.2 :Sb, and TiO.sub.2 /Ag/TiO.sub.2. However, several disadvantages are generally brought about by using such an oxide material in forming a transparent electrode. For example, glass is typically used as a base plate or substrate on which a desired transparent electrode is formed and alkali ions, such as Na.sup.+ and K.sup.+, are easily diffused into the transparent electrode formed on the glass substrate thereby forming acceptor levels therein. As a result, donors are captured by these acceptor levels to lower the conductivity of the transparent electrode, thereby deteriorating device characteristics. In addition, such reduction in conductivity occurs locally so that irregularities are formed in the conductivity of the transparent electrode, thereby forming a non-uniform characteristic. This problem becomes prominent when a transparent electrode of larger area is to be formed, and resulting devices will be poor in reliability and durability.
In particular, in the case of an oxide family transparent electrode, it is rather chemically unstable and it gradually becomes decomposed, for example, due to water content in the atmosphere so that its conductivity becomes lower, thereby deteriorating the characteristics required for an intended optical device. As described previously, in the case of an a-Si:H solar cell, for example, when a transparent and electrically conductive film is formed in the plasma decomposition atmosphere of SiH.sub.4, since it is placed in a reducing atmosphere, an oxide family transparent electrode, such as ITO, is inevitably reduced. As a result, reduced metal indium is diffused into the film of a-Si:H, thereby deteriorating the characteristics as a solar cell. Such an argument generally holds true for other oxide family transparent electrodes.